I am having a clear out of thread protectors I've collected. I think used difficult to tell.
4 X 1/2UNF Blued steel 16.3mm outside diameter, 12.5mm long, knurled although looks incomplete on one of them.
1/2UNF Blued steel 21.2mm outside diameter, 12.5mm long, think this was to replace the plastic one on a Sako Quad varmint
5/8UNF Stainless steel 21.2mm outside diameter, 15.5mm long
All £7 delivered (paypal please) PM me